1. Field of the Invention
The present invention relates to an active filter device for suppressing higher harmonic current components of the power supply system.
2. Description of the Related Art
FIG. 2 is a circuit diagram of the construction of a conventional active filter device of the above-mentioned type disclosed in Mitsubishi Electric Corporation Technical Report vol.62.No6, 1988 p15-p20 and shows a connection between such a device and a controlled power supply system.
Referring to FIG. 2, a system power supply 1 supplies power to a load to be compensated 2 which is connected to the system power supply 1. The load to be compensated 2 comprises a higher harmonic supply 3, such as an inverter, and a capacitative load 4, such as a phase lead capacitor, or the like. A current transformer 5a detects the load current I.sub.L flowing into the load to be compensated 2. An active filter device 6 outputs the compensation current I.sub.C which has the same level and the opposite phase of a higher harmonic current component contained in the load current I.sub.L on the basis of the output, from the current transformer 5a.
The active filter device 6 comprises components within the frame indicated by the one-dot chain line of FIG. 2. More specifically, the active filter device 6 comprises: a current transformer 5b for detecting components from the compensation current I.sub.C ; an inverter transformer 7 for permitting the compensation current I.sub.C to flow from the power system formed of the system power supply 1 and the load to be compensated 2; a self-excited inverter 8 formed of a plurality of inverter units so as to drive the inverter transformer 7 and to convert the DC voltage charged on a DC voltage supply capacitor 9 into the AC voltage; a reference compensation current detection circuit 101 for detecting the reference compensation current I.sub.C * based on the load current I.sub.L (the details will be mentioned below); an adder-subtracter 11 for calculating the difference .DELTA.I between the reference compensation current I.sub.C * and the output I.sub.C - from the current transformer 5b; a current control circuit 12 for calculating the reference voltage V* based on the above-mentioned difference .DELTA.I and the output from a capacitor voltage control circuit 13; the capacitor voltage control circuit 13 for controlling the voltage of the DC voltage supply capacitor 9; and a PWM (Pulse Width Modulation) control circuit 14 for driving the self-excited inverter 8 based on the output from the current control circuit 12.
The above-noted reference compensation current detection circuit 101 comprises components within the frame indicated by the broken line of FIG. 2. More specifically, the reference compensation current detection circuit 101 comprises: a PLL circuit 101a for detecting the angular velocity .theta. (the frequency of the fundamental wave) in synchronism with the system power supply; a 3.phi./2.phi. conversion circuit 101b for converting an AC component of the three-phase signal (i.sub.La, i.sub.Lb, i.sub.Lc) detected by the current transformer 5 into a DC component of the two-phase signal (i.sub.d, i.sub.q) on the basis of the angular velocity .theta.; a filter circuit 101c for smoothing the two-phase signal (i.sub.d, i.sub.q) which is output from the 3.phi./2.phi. conversion circuit 101b; and a 2.phi./3.phi. conversion circuit 101d for converting the two-phase signal (id, i.sub.q) smoothed by the filter circuit 101c into the three-phase signal on the basis of the angular velocity .theta. so as to output the converted signal as the reference compensation current I.sub.C *. The reference numeral (3) in FIG. 2 indicates a signal line of the three-phase signal.
A description will now be given of the operation of the active filter device 6. The active filter device 6 is connected parallel to the load to be compensated 2 so as to detect fault current components such as a higher harmonic reactive current, or the like, contained in the load current I.sub.L and to allow the compensation current I.sub.C which has a phase opposite to that of a fault current to flow into the active filter device 6, thereby canceling the fault current component on the power supply. An explanation will be given in more detail in the case where the load current I.sub.L is supplied to the load to be compensated 2 from the system power supply 1. The current transformer 5 first detects the load current I.sub.L, which is then supplied to the reference compensation current detection circuit 101. The reference compensation current detection circuit 101 determines a current which compensates the higher harmonic of the load current I.sub.L, that is, the reference compensation current I.sub.C * (the details will be mentioned below) on the basis of the load current I.sub.L. The adder/subtracter 11 calculates the difference .DELTA.I between the reference compensation current I.sub.C * and the component I.sub.C - of the current I.sub.C (compensation current) flowing into the inverter transformer 7, which difference .DELTA.I is then output to the current control circuit 12. The current control circuit 12 calculates the reference voltage V* based on the difference .DELTA.I and the output from the capacitor voltage control circuit 13. The PWM control circuit 14 compares the reference voltage V* with a triangular wave transfer signal which is to be used for the pulse width modulation (PWM) and performs the modulation to drive the inverter units constituting the self-excited inverter 8 so that the difference .DELTA.I can be small. The output voltage of the active filter device is thus controlled. It is noted that the output waveform (average) of the inverter units is indicated by the sine wave by performing the pulse width modulation using the triangular wave transfer signal.
The self-excited inverter 8 serving the function of the major portion of the active filter device is comprised of a plurality of inverter units which are connected in series to each other via the inverter transformer 7. The respective inverter units are controlled by the PWM control circuit 14 so as to generate the voltage required for allowing the compensation current I.sub.C to flow into the inverter transformer 7. The inverter 8 thus serves the function of converting the DC voltage charged on the DC voltage supply capacitor 9 into the AC voltage so that it can be a voltage supply for generating the AC voltage E.sub.I.
As described above, the reference compensation current detection circuit 101 detects the reference compensation current I.sub.C * based on the load current I.sub.L.
The operation of the reference compensation current detection circuit 101 will now be given in detail.
The PLL circuit 101a detects the angular velocity .theta. (the frequency of a fundamental wave) in synchronism with the system voltage. The 3.phi./2.phi. conversion circuit 101b converts the load current I.sub.L (the respective currents of the three phases are indicated by i.sub.La, i.sub.Lb, i.sub.Lc) detected by the current transformer 5 into the two-phase signal (i.sub.d, i.sub.q) of the dq coordinate system on the basis of the angular velocity .theta.. The two-phase signal (i.sub.d, i.sub.q) is expressed by the following equations: EQU i.sub.d =(2/3) [i.sub.La sin .theta.+i.sub.Lb sin {.theta.-(2/3).pi.}+i.sub.Lc sin {.theta.-(4/3).pi.}] EQU i.sub.q =(2/3) [i.sub.La cos .theta.+i.sub.Lb cos {.theta.-(2/3).pi.}+i.sub.Lc cos {.theta.-(4/3).pi.}]
wherein i.sub.d indicates a phase component different from the reference angular velocity .theta. and i.sub.q represents an in-phase component of the angular velocity .theta..
The dq conversion is conducted to determine the DC component of the angular velocity .theta.. In other words, the above equations are calculated so as to convert the three-phase AC into the two-phase AC based on the respective AC components and further to convert the stationary coordinate system into the rotating coordinate system on the basis of the angular velocity .theta.. This conversion will be further explained with reference to FIGS. 4A-4C. The three-phase AC i(i.sub.u, i.sub.v, i.sub.w) on the three-phase stationary coordinates u, w and v shown in FIG. 4A are converted into the two-phase AC i(i.sub..alpha.,i.sub..beta.) on the two-phase stationary coordinates .alpha. and .beta. shown in FIG. 4B. Such a two-phase current i rotates on the stationary coordinates .alpha. and .beta. which are thus converted into the two-phase rotating coordinates d and q illustrated in FIG. 4C, thereby allowing the two-phase current i to become stationary on the two-phase rotating coordinates d and q. That is, the two-phase signal (i.sub.d, i.sub.q) on the two-phase rotating coordinates d and q is DC. The foregoing equations are calculated to execute the above-mentioned conversions. The DC component of the angular velocity n.theta. of the higher harmonics can be found in a manner similar to that of the angular velocity .theta. of the fundamental wave.
The thus-obtained output i.sub.d and i.sub.q from the 3.phi./2.phi. conversion circuit 101b correspond to a phase component different from a signal indicative of the angular velocity .theta. and an in-phase component of the angular velocity .theta., respectively. Where the phase difference between the rotation axis upon which the conversion of the rotation coordinate system is based and the vector (i.sub.d, i.sub.q) is indicated by .phi., the relationship between the two components id and iq is illustrated in FIG. 5.
The filter 101c is a circuit for smoothing the two-phase signal (i.sub.d, i.sub.q). A high-pass filter (HPF) is used to extract the higher harmonic component i.sub.d other than the fundamental wave.
The 2.phi./3.phi. conversion circuit 101d then re-converts the two-phase signal into a three-phase signal on the basis of the output from the filter 101c and the angular velocity .theta.. Such a three-phase signal is expressed by the following equations: EQU I.sub.ca *=i.sub.d cos .theta.+i.sub.q sin .theta. EQU I.sub.cb *=i.sub.d cos (.theta.-(2/3).pi.)+i.sub.q sin (.theta.-(2/3).pi.) EQU I.sub.cc *=i.sub.d cos (.theta.=(4/3).pi.)+i.sub.q sin (.theta.-(4/3).pi.)
In a manner described above, the reference compensation current detection circuit 101 outputs the reference compensation current I.sub.C * (the compensation volume for removing higher harmonics). The inverter circuit 8 drives the inverter transformer 7 to allow it to generate the compensation current I.sub.C on the basis of the reference compensation current I.sub.C *.
A specific waveform is illustrative of the above operation by way of example. FIG. 3 illustrates the waveforms obtained from the operation of the assumed rectifier load. The load current I.sub.L shown in FIG. 3 is detected by the current transformer 5. The load current I.sub.L can be divided into the fundamental wave component I1 (the waveform indicated by the broken line) and the higher harmonic I.sub.H (indicated by the hatched portion). For removing the higher harmonic I.sub.H, the active filter 6 detects it (the reference compensation current I.sub.C *) by the reference compensation current detection circuit 101 and actuates the self-excited inverter 8 so as to allow the current I.sub.C which has a phase opposite to that of the higher harmonic I.sub.H to flow from the supply system to the active filter 6. Thus, the current I.sub.H can be canceled by the current I.sub.C, thereby permitting the sine wave current I.sub.S including only the fundamental wave component to flow into the power supply.
The conventional active filter device is constructed as described above so that higher harmonics other than the fundamental wave can be removed. However, such a device presents the following problems. A phase margin is dissipated for the higher order of harmonics because of the idle time spent by the control system of the active filter device 6, thus jeopardizing stable control.
Further, as illustrated in FIG. 2, when the capacitative load 4 is contained in the load to be compensated 2, antiresonance (parallel resonance) is caused between the load 2 and the power supply reactance, thereby increasing the possibility of intensifying the higher harmonics (the active filter device compensates for the antiresonant frequency component contained in the load current, thereby making the control system unstable). In other words, when the active filter device 6 compensates for the higher harmonics at the above-mentioned antiresonance point, a phase margin is dissipated because of a sharp change in the phase at the antiresonance point and the idle time spent by the control system of the active filter device, thus enhancing the diverged control and making the device inoperative.
In order to overcome the above drawbacks, it is necessary to use a passive filter in order to absorb the higher harmonic current in the vicinity of the antiresonant frequency, and accordingly, such a system is constructed of a combination of an active filter device and a passive filter, thus increasing the price of the system.